1. Field of the Invention
The present invention relates to a focus detecting apparatus of a camera which has a plurality of object distance measuring zones.
2. Description of Related Art
Recent single-lens reflex cameras usually have an automatic focusing device. A known automatic focusing device in a conventional single-lens reflex camera is a focus detecting apparatus which detects whether beams of light are focused on an object to be photographed and a lens driving device which moves a photographing lens (focusing lens) to a focused position in accordance with the result of the detection of the focus detecting apparatus.
FIG. 25 is a schematic sectional view of a known single-lens reflex camera having a focus detecting apparatus, taken along an optical axis. In FIG. 25, a camera body 1 has an interchangeable lens 2 incorporated therein having a group of photographing lenses which will be referred to as a photographing lens group L hereinafter. An object distance measuring unit 160, which constitutes a focus detecting apparatus, is provided below a main mirror 3 provided in the camera body 1, at a position optically equivalent to a film plane 4. The main mirror 3 has a half mirror portion at its center, so that the beams of light which are transmitted through the photographing lens group L and which are incident on the main mirror 3 are partly transmitted through the half mirror portion thereof.
The beams of light transmitted through the half mirror portion are reflected by an auxiliary mirror 6 and are incident upon the object distance measuring unit 160. Positioned above the object distance measuring unit 160 is a light intercepting plate 211, which intercepts beams of light which are not used to detect the object distance, and an opening 212 through which the beams of light to be used for the detection of the object distance are transmitted.
The beams of light which are reflected by the main mirror 3 are transmitted through a focusing glass 7 and then reflected by a pentagonal prism 9, so that the beams of light are introduced into a finder 8.
The finder 8 has an object distance measuring zone 177 at a center portion of a finder field of view 176, as shown in FIG. 30. The object in the object distance measuring zone 177 is detected to adjust the focus.
As shown in FIG. 26, the object distance measuring unit 160 has a field mask 163, a condenser lens 164, an aperture mask 165, separator lenses 166 and 167 which constitute an image separating optical system to split an aerial image of an object into two images, and a CCD image sensor 168 which constitutes an object distance measuring sensor.
The field mask 163, condenser lens 164, aperture mask 165 and separator lenses 166 and 167 constitute an automatic focusing optical system 169 which, in turn, constitutes a focus detecting optical system together with the CCD image sensor 168.
The field mask 163 of the automatic focusing optical system 169 is provided in the vicinity of a film equivalent plane 170. The film equivalent plane 170 is optically conjugate with the object 162 with respect to the photographing lens group L, so that when the photographing lens group L is located in the focused position, an aerial image 171 of the object 162 in focus is formed on the film equivalent plane 170.
The condenser lens 164 and the aperture mask 165 split the beams of light passing the right and left sides of the photographing lens group L into two luminous fluxes. The separator lenses 166 and 167 are located in a relationship conjugate to the photographing lens group L through the condenser lens 164.
The separator lenses 166 and 167 are usually located side by side in parallel with the length of the film plane, as shown in FIG. 27, so that imaginary opening areas 174 and 175 of an exit pupil of the photographing lens group L mounted to the camera body 1 can be viewed through the measuring zone 172 which is optically conjugate with the center measuring zone 177 (FIG. 30) in the finder field of view 176. The beams of light which are transmitted through the opening areas 174 and 175 are incident upon the seperator lenses 166 and 167, so that two images 171a, 171a of the aerial image 171 formed on the film equivalent plane 170 are reformed on the CCD image sensor 168 by the seperator lenses 166 and 167.
Assuming that the distance between the reformed images 171a and 171a are in focus (FIG. 28(a)), the distance between peak values of image signal S is l.sub.0, as shown in FIG. 29(a). When the focal point of the photographing lens group L is located before the in-focus position shown in FIG. 29(a), as shown in FIG. 28(b), the distance is decreased, so that distance l.sub.1, between the peak values of the image signal S, is smaller than l.sub.0. However, when the focal point of the photographing lens group L is located behind the in-focus position shown in FIG. 28(a), the distance is increased, so that distance l.sub.2, between the peak values of the image signal S, is larger than l.sub.0, as shown in FIG. 28(c).
The change of the image distance is substantially proportional to the amount of defocus of the photographing lens group L and Accordingly, in a conventional single-lens reflex camera, the distance of the images formed on the CCD image sensor 168 is detected to arithmetically process the same in order to determine the direction and amount of defocus of the photographing lens group L, thereby moving the photographing lens group L to the focused position (in-focus position).
As can be seen in FIG. 30, for example, when a photographer has determined a composition so that the desired object 162 is located in the center measuring (focusing) zone 177 of the finder field of view 176, a focusing lens of the photographing lens group L is moved to the focused position by a focusing lens driving motor (not shown) and a focusing lens driving mechanism (not shown), where an in-focus picture can be taken.
In the focus detecting device mentioned above, since the measuring zone 177 is located in the center area of the finder field of view 176, the photographing lens is focused on the object 162.
However, a case exists where a desired object should be located in the circumferential area, rather than the center area of the finder field of view.
Furthermore, there is also a case where two human figures can not be placed in the center area of the finder field of view 176, namely in the measuring zone 177.
To this end, a conventional camera has a so-called focus lock device in which the focusing is effected while the object is placed in the center measuring zone 177 of the finder field of view 176, and then the focus is locked. In this state, when a desired framing is effected, a picture in which the desired object is placed in the circumferential area and the camera is focused on the object 162, as shown in FIG. 31 can be taken.
However, in the conventional focus detecting device, the focusing is effected (by moving the photographing lens) once the object is placed in the center of the finder field of view 176, and the focus is locked to fix the photographing lens group L. After that, the composition is redetermined and the releasing is effected. Due to these troublesome operations, it takes a great deal of time to take a picture.
To this end, there is also known a multi-focus detecting apparatus in which two circumferential measuring zones 177a and 177b are provided to surround the measuring zone 177 of the finder field of view 176, so that whether or not the camera is focusd on the object placed on the circumferential zones 177a and 177b is also detected, as can be seen in FIG. 30.
Such a multi-focus detecting apparatus usually has a first center automatic focusing optical system 169, a pair of second and third automatic focusing optical systems 211 and 212, and first, second and third CCD image sensors 168, 241 and 242, as shown in FIG. 32.
The first automatic focusing optical system 169 has a condenser lens portion 222 and a separator lens portion 232. The second automatic focusing optical system 211 has a condenser lens portion 221 and a separator lens portion 231. The third automatic focusing optical system 212 has a condenser lens portion 223 and a separator lens portion 233.
Numerals 177', 177a', 177b' designate fields of view of the center measuring area and the circumferential measuring areas of the first, second, and third automatic focusing optical systems 169, 211 and 212 and are substantially conjugate with the measuring zones 177, 177a and 177b of the finder field of view 176, respectively. The CCD image sensors 168, 241 and 242 are formed on the same integrated circuit board 251.
Upon taking a picture of a human figure, a photographer usually unconsciously looks at the persons face through the finder. This is because the face reflects a state of mind of human being. Usually, the face of a human figure in a portrait is located slightly above the center of the finder field of view rather than the center.
However, in the conventional focus detecting apparatuses, the center measuring zone 177 is provided at the center of the finder field of view 176. As a result, upon taking a picture of a human figure, the center measuring zone 177 does not often correspond to the face of the human figure, as shown in FIG. 30. Therefore, after the center of the finder field of view 176 is located to correspond to the face, the focusing is effected and locked. After that, the framing is effected (FIG. 30). Therefore, a photographer tends to concentrate his or her attention to agreement of the face of the human figure with the measuring zone 177 and miss a chance of taking a picture that shows a change of expression.
Furthermore, in the conventional multi-focus detecting apparatus, since three CCD image sensors 168, 241 and 242 are arranged in the form of a generally H-shape, as shown in FIG. 33, it is necessary to bend a single transfer register 252 which reads data of the three image sensors 168, 241 and 242, as shown in FIG. 33, resulting in an increased transfer distance.
The quantity of charge accumulated by the CCD sensors 168, 241 and 242 depends on the intensity (brightness) of the luminous flux of the object. It is also known to provide a monitor sensor 168m in the vicinity of the center CCD image sensor 168 to detect the amount of light incident thereon, to control the integration time (time of accumulation of the charges).
In the known detecting mechanisms mentioned above, in which the integration time is controlled by a mean value of the brightnesses detected by the measuring zones 177, 177a and 177b, or by the brightness of the center measuring zone 177, if there is a large difference in brightness between the measuring zones 177, 177a and 177b, the quantity of charge accumulated in the CCD image sensor corresponding to the measuring zone having a high brightness exceeds a saturation value, so that detection can not be effected, or a sufficient charge can not be accumulated in the CCD image sensor corresponding to the measuring zone having a low brightness, so that the detection can not be effected.
The CCD image sensor used in a conventional focus detecting apparatus, as mentioned above, has a light receiver having an array of photoelectric converting elements, an integral portion which integrates the signal charge converted by the light receiver, and a charge transfer portion which reads the integrated signal charge. The signal charge integrated by the integral portion is transferred to the charge transfer portion, and is then successively transferred from the charge transfer portion by at least a two-phase transfer of transfer pulses which are generated at a constant interval. These transfer pulses are usually outputted periodically, so that the signal charge is transferred during the period of outputting.
No accumulated control pulse is output during the period of outputting of the transfer pulses to prevent the signal charge from being transferred from the integral portion to the charge transfer portion. This is because if the accumulated control pulse is output when the charge transfer portion has an electic charge therein, the signal charge is mixed with the accumulated control pulse in the charge transfer portion, so that the signal no longer functions as a data signal.
To avoid this, even if the brightness of an object is high, so that the integral portion is saturated, the integral operation cannot be completed until the output time of the transfer pulse ends. As a result, when the brightness of an object is high, no precise focus detection can be effected.
To solve the problem mentioned above, it is known to make the prohibition period of outputting the accumulated control pulse variable (e.g. Japanese Unexamined Patent Publication No.60-121409). However, such a varying means makes the circuit construction and the control operation thereof complex.